Microdialysis provides a window on the brain. It permits measurements to be made of the extracellular concentration of chemicals whose primary role is an extracellular function, for example neurotransmitters. It has been widely adopted for investigations in animal models of disease and trauma and indeed in humans suffering trauma. The technique, as powerful as it is, has one major limitation - it cannot measure rapid changes that are relevant to phenomena such as spreading depolarization and behavior. There is a large gap in the time regime over which scientists can measure extracellular concentrations of the neurotransmitters dopamine (few nanomolar basal levels) and serotonin (sub-nanomolar basal levels) in the sub-minute to five minute range. A complementary technique, fast scan cyclic voltammetry measures changes in concentrations over the sub-second to several second timeframe. Based on our sound theoretical insight and technical improvements, we propose to improve the timescale of microdialysis to cover this important range. We will address three types of experiments with the new capability. (1) Are serotonin oscillations we have seen in preliminary work due to spreading depolarization? (2) What changes in the stress/anxiety-related neurotransmitter serotonin (5-hydroxytryptamine) are seen during the time an animal is making a decision in a stressful situation? (3) Can microdialysis be used to distinguish dopamine release upon an animal's receiving a reward from dopamine release upon the animal's receiving a cue associated with the reward?